Screw tightening structures have been widely known which include: (a) an indented concave recess provided on a screw, and mating convex sections provided on a screw tightening tool; (b) the recess and the mating convex sections including three or more torque transfer portions, respectively, which protrude outward at equiangular intervals for concentrically mating engagement; and (c) wherein rotating the screw tightening tool causes the torque transfer portions of the mating convex sections to transfer a tightening torque to the screw via the torque transfer portions of the recess (see Patent Publications 1 and 2).
FIG. 10 shows an example of such a screw tightening structure in a case wherein a recess 102, provided on a head of a screw 100, and a mating convex sections 106 of a screw tightening tool 104 include three torque transfer portions 108 and 110, respectively, all of which are located concentrically with center lines O1 and O2 of the screw 100 and the screw tightening tool 104, respectively. With the recess 102 and the mating convex sections 106 held in mating engagement with each other in a concentric relation, further, the screw tightening tool 104 is caused to rotate clockwise about the center line O2. This causes outer circumferential edge portions of the torque transfer portions 110 of the three mating convex sections 106 to be brought into abutting contact with side wall face of the torque transfer portions 108 of the recess 102, respectively, with tightening torque being transferred to the screw 100 via force application points Q representing resulting abutment portions. The force application points Q are present at opening end portions (on positions of a paper sheet of FIG. 10(a)) at which the recess 102 is open at an end face 112 of the head of the screw 100. The screw tightening tool 104 includes a screw driver or a bit. FIG. 10(a) is a transverse cross-sectional view taken on a plane perpendicular to the center lines O1 and O2 and corresponds to a cross section taken on line XA-XA in FIG. 10(b). FIG. 10(b) is a longitudinal cross-sectional view involving the center lines O1 and O2 and corresponds to a cross section taken on line XB-XB in FIG. 10(a).